There are known many alternative interconnected suspension systems which have the ability to passively differentiate between different modes of wheel motion with respect to the vehicle body and therefore provide a variety of alternatives in functionality. For example, the Applicant's U.S. Pat. No. 6,270,098 provides a pressure balancing “load distribution” unit between two pairs of diagonally interconnected double acting wheel rams. This system provides different heave, roll and pitch stiffness rates with zero warp stiffness and different damping rates in all four base suspension modes (heave, roll, pitch and warp). This system supports the weight of the vehicle, so as the loads on the vehicle change, or as the fluid temperature changes, the volume of fluid in each of the six volumes in the system must be adjusted. Also, as the six volumes in the system can in some load conditions, all be at different pressures, there is the possibility for fluid to leak across seals, which also requires fluid volume adjustments to be made to maintain the correct vehicle attitude. This requires a high pressure fluid source, sensors, control electronics and valves, making the cost of the system relatively high for a passive system.
An example of a passive interconnected hydraulic system providing many of the benefits of the above (U.S. Pat. No. 6,270,098) without the cost of a high pressure control system can be found in the applicant's PCT/AU03/01637 (Australian patent application number 2003291836). This lower cost system can, in some embodiments, provide high roll stiffness with low warp stiffness, negligible heave stiffness and a different stiffness rate in pitch. It also provides high roll damping and pitch damping with lower, more comfortable and isolating heave damping. As this hydraulic system does not provide significant heave stiffness, separate support springs are required, giving the advantage that all six system volumes can operate at a common pressure, negating the need for a high pressure control system. However, when pitch stiffness is provided, the system has six volumes, a single load distribution unit which requires two accumulators and can be difficult to package, or if split into two halves, requires additional hydraulic conduits in the vehicle. The layout of the conduits of the system can also give rise to fluid acceleration effects.
An example of a system having just roll and/or pitch damping can be found in U.S. Pat. No. 5,486,018 and U.S. Pat. No. 6,024,366. The system in each of these documents uses a device between a pair of wheel damping rams, each wheel damping ram having a damper valve in its piston to provide double-acting damping but make the ram single-acting (i.e. there is only one fluid port). The device provides for independent levels of damping for in-phase (i.e. heave) and out of phase (i.e. roll and/or pitch) motions. However this system does not provide significant stiffness in any mode, so in addition to the need for support springs, generally anti-roll bars will be required for a good balance between bounce and roll stiffness. Additionally, as the wheel rams are effectively single acting (having only one fluid port) the amount of damping that the device can provide is limited. There are improvements made to the system to combat this problem, which can be found in Japanese patent office publication number 11291737, but these add to the complexity of the system by providing more plumbing and spool valves.
With the aforementioned in mind, it is therefore an object of the present invention to provide a hydraulic system for a vehicle suspension that alleviates at least one of the disadvantages of such known vehicle suspension systems.
It is a preferred object of the present invention to provide a hydraulic system exhibiting roll stiffness, pitch stiffness, roll damping, pitch damping and heave damping, all of which can be configured and tuned substantially independently from each other to enable optimisation of each parameter.